Core 2 Duo is a dual core processor without hyperthreading
Core 2 Quad is a quad core processor without hyperthreading
Core i3 is a dual core processor with hyperthreading
i5 is a quad core processor without hyperthreading
i7 is a quad/hexa core processor with hyperthreading

Mishaal, as you may have picked up from the answers, thre isn’t a single MAIN difference. It’s like comparing cars from different makes, models and made in different years.
The Core 2 range are older, Core i3-i7 are the latest.
Very generally, new CPUs are faster than old ones, but as there is a huge range of speeds, this is not true for each and every combination (e.g. some top-of-the-range Core 2 Quads will be faster than the i3).
You can see a review of the CURRENT processor range at http://www.intel.com/content/www/us/en/processor-comparison/compare-intel-processors.html?select=desktop
The fastest processor, as others have said, would be the top of the range i7.

i7 is surely the fastest in comparison to all the other processors. I’m sure we’d all unanimously recommend that you go for a computer with the fastest available processor, but usually, cost is also a factor.
It depends on how you wish to use your computer. If you use your computer for general use, like e-mail, social networking, document editing, I’d say a core 2 duo should be fine all the way up to an i3. If you use photoshop or do 3D modeling, an i5 would be a minimum requirement for optimal performance. And of course, if you’re a heavy gamer, tend to run 5-10 processor-hungry programs at the same time (though that has more to do with RAM, than processing), an i7 is your processor.

The difference mostly is within the details I’m afraid. The only distinct difference is that the Core-i series is the successor to the Core 2 family. But that doesn’t rule out that a Core 2 Quad could be faster than a Core i3.

The “2” basically stands for 2. generation wherein the “i” is considered third generation. However, this has to be distinguished that the Core-i series is currently in it’s own third generation – generation “3.3” so to speak.

Also, each processor comes in multiple versions with different frequencies (Gigahertz).

While this seems to be the main difference it doesn’t give any indication about the processing power of the CPU.

a upper-range Core 2 Duo processor can be better than a Core 2 Quad
a upper-range Core 2 Quad processor can be better than a Core i5
a upper-range Core i5 processor can be better than a Core i7
a mobile Core i7 even with 4 cores might be slower than a Core i5 (Desktop)

Adding to this each processor series saw several updates over the time they have been on the market. The Core i series is currently in it’s third generation. So a first generation Core i3 with 3.1GHz might not be as good as a third generation Core i3 with 2.9GHz although numbers indicate it has 200MHz higher speed

All the Core i7 processors are Native Quad Cores (4 cores), which means that they actually have 4 cores on a single die, older Core 2 Duo processors like the Q6600 (2×2 = 4 cores) had two cores on a single die. Core i7 processors support HyperThreading which means that each of the 4 cores can process 2 threads simultaneously, so you get 8 virtual cores. They also have 256 KB L2 cache per core and a shared 8 MB L3 cache which has proven to be much better than any cache configuration in Core 2 Duo. cire i5 CPUs have 3MB to 6MB of L3 cache, while i7 processors have 8MB to 15MB. You’ll find Intel HD graphics 3000 and 4000 on Core i7, while Core i5 features one of all four versions of Intel HD graphics, depending on the part number. Integrated graphics saves power, since there won’t be an extra graphics chip on your laptop or desktop’s motherboard using power.http://www.pathikshah.com/blog/intel-core-i7-vs-intel-core-2-duo-the-difference/

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